The invention relates to a system for transmitting an n-level data signal at a given symbol rate 1/T. This system including a data transmitter with a data signal source, a transmission channel and a data receiver with an equalizer of the decision feedback type, which equalizer comprises a feedforward filter connected between the input of the data receiver and a first input of a difference circuit, a symbol decision circuit connected to the output of the difference circuit, and a feedback filter connected between the output of the symbol decision circuit and a second input of the difference circuit, in which the linear part of the transmission path between the output of the data signal source and the first input of the difference circuit can be described by a linear signal transformation .
Such a system is generally known and is described, for example, in the book "Digital Communications" by J. G. Proakis, McGraw-Hill, 1983, Chapter 6, Section 6.5, pp. 382-386. In such systems the feedforward filter belonging to the equalizer provides for suppression of noise and cancellation of pre-cursive intersymbol interference (ISI), whilst post-cursive intersymbol interference (ISI) is cancelled with the aid of the feedback filter synthesizing a replica of this interference on the basis of the symbol decisions already formed, by which replica is subtracted from the output signal of the feedforward filter. In the system known from the book by Proakis the equalizer is arranged for forming at the input of the symbol decision circuit an estimate of a data signal generated by the data transmitter. Normally, this estimate relates to the original n-level data signal, but in the case when the data transmitter includes a linear encoder, it is likewise possible to have this estimate relate to the output signal of the encoder and reconstruct in the data receiver the original n-level data signal from the symbol decisions formed then. The latter possibility occurs, for example, in ISDN transmission systems in which pseudo-ternary transmission codes are used, compare the article "A Baud-Rate Line-Interface for Two-Wire High-Speed Digital Subscriber Loops" by C. A. Ehrenbard and M. F. Tompsett, Proc. GLOBECOM 1982, Miami, USA, pp. D.8.4.1-D.8.4.5, in which the use of a bipolar transmission code is described.
In strongly dispersive transmission channels the output signal of the feedforward filter shows a strongly post-cursive intersymbol interference (ISI). Since the feedback filter has to synthesize a replica of this post-cursive ISI, erroneous symbol decisions applied to the feedback filter will more seriously affect subsequent symbol decisions according as the transmission channel is more dispersive. This undesired continuing influence of erroneous symbol decisions is known as error propagation and entails a degradation of the transmission quality, as appears, for example, from FIG. 6.5.2 on page 386 of the above book by Proakis.